The present invention relates to an improved nuclear reactor having a calandria in the upper portion thereof, and to a deep beam head adapted for use on a nuclear reactor to seal the top of the reactor pressure vessel. In nuclear reactors, a core is supported in a pressure vessel and a coolant, typically water at critical temperature and pressure is circulated upwardly through the core and outwardly from the pressure vessel to provide power. The coolant flows upwardly through the core and then transversely to the outlets. The reactor also includes control rod assemblies and water displacement assemblies, which are movable into and out of the core to control the same.
The use of a calandria structure in the upper region of the pressure vessel has been proposed, the calandria having an upper support plate, lower support plate and a series of hollow members therebetween, with the rod drive mechanisms operating through the hollow members. The coolant passes upwardly into the calandria through apertures in the lower support plate and is directed transversely about the hollow members to the outlet nozzles, while the control rods are protected by their shielding with the walls of the hollow members.
Such a system is described in copending application Ser. No. 490,099 filed Apr. 29, 1983 in the names of Luciano Veronesi, et al., entitled "Nuclear Reactor", which application is assigned to the assignee of the present invention, and which application is incorporated by reference herein. In the improved reactor described in said pending application, a yoke member carrying a plurality of control rods is movable in the space between the calandria and the core. In such systems, however, a dome-shaped head or cover for the pressure vessel is used with an open area present between the calandria and the dome-shaped head. This large area of unused space between the calandria and the dome-shaped head causes certain technical difficulties that are overcome by use of the present invention.